1. Field of the Invention
The present invention relates to a photoelectric conversion apparatus having a light-receiving element formed in a semiconductor substrate, and an image sensor.
2. Related Background Art
In recent years, in the field of linear photoelectric converters, CCDs using a reduction optical system, and equal-magnification image sensors that mount a plurality of semiconductor photosensor chips have been extensively developed.
A light-receiving element in such photoelectric conversion apparatus normally comprises a photodiode formed by a pn junction of semiconductors. For example, as disclosed in Japanese Patent Application Laid-Open No. 55-154784, a structure in which a region having the same conductivity type as that of a substrate and higher impurity concentration than the substrate is formed in a substrate surface portion where no pn junction is formed so as to reduce the dark currents produced on the substrate surface has also been proposed. On the other hand, as a light-receiving element for a linear photoelectric conversion apparatus, various structures such as a structure which reduces the junction capacitance formed by a pn junction, as disclosed in, e.g., Japanese Patent Application Laid-Open No. 61-264758, a structure which reduces the dark currents resulting from the scribed chip edge, as disclosed in Japanese Patent Application Laid-Open No. 1-303752 and so on, have been proposed. A photodiode formed by a pn junction is generally known.
However, when such light-receiving element is applied to an amplifier type photoelectric conversion apparatus which accumulates photocarriers in a pn junction, and reads out a signal voltage using a charge-voltage conversion means, high sensitivity cannot be obtained.
In case of the amplifier type photoelectric conversion apparatus, the light output is given by: EQU Vp=Qp/Cs (1)
where Qp is the amount of charge accumulated on the pn junction, and Cs is the capacitance of the photoelectric conversion portion.
In case of, e.g., an amplifier type photoelectric conversion apparatus comprising a photodiode, MOS source-follower circuit, and reset MOS circuit, the capacitance Cs of the photoelectric conversion portion is given by: EQU Cs=Cpd+Ca (2)
where Cpd is the pn junction capacitance of a pn photodiode, and Ca is another capacitance connected to the photoelectric conversion portion, which capacitance includes the gate capacitance of a MOS transistor which forms the MOS source-follower circuit, and the source/well junction capacitance, source/gate overlapping capacitance, wiring capacitance, and the like of a MOS transistor that forms the reset MOS transistor.
Hence, in order to realize high sensitivity, photocarriers must be effectively accumulated, and the capacitance of the photoelectric conversion portion where the photocarriers are accumulated must be minimized.
However, in a contact image sensor which uses a photodiode obtained by forming in a semiconductor substrate a region having a conductivity type opposite to that of the semiconductor substrate, since the pixel pitch at a resolution of, e.g., 300 dpi is around 84.7 microns, a pn junction having an area nearly equal to the opening is required to effectively read out photocarriers, but then the pn junction capacitance Cpd of the photodiode portion in equation (2) increases.
On the other hand, when the pn junction area is decreased to reduce the pn junction capacitance Cpd of the photodiode portion, photocarriers accumulated on the pn junction region decrease.
Japanese Patent Application Laid-Open No. 61-264758 discloses a technique for forming an accumulation portion having an annular shape or partially cutaway annular shape so as to reduce the junction capacitance of the accumulation region.
However, when a region having the same conductivity type as that of the substrate and higher impurity concentration than that of the substrate is formed in the substrate surface for the purpose of suppressing the dark currents produced on the substrate surface, as disclosed in Japanese Patent Application Laid-Open No. 55-154784, the depletion layer narrows down around the junction, and the circumferential length dependence of the pn junction capacitance increases, as shown in FIG. 2 of Japanese Patent Application Laid-Open No. 55-154784.
Hence, the structure disclosed in Japanese Patent Application Laid-Open No. 61-264758 can hardly attain high sensitivity, since the capacitance value of the pn junction cannot be sufficiently reduced due to an increase in circumferential length although the pn junction area decreases.
On the other hand, along with development of recent micropatterning techniques of semiconductor elements, a technique that uses Ti, TiN, or the like as a barrier metal, i.e., a wiring material that contacts a semiconductor diffusion layer is normally used.
Hence, in order to obtain a satisfactory ohmic contact with a p-type diffusion layer, the impurity concentration of the semiconductor diffusion layer must be higher than that when Al is used as a wiring material.
However, in the prior art, when the impurity concentration of a p-type region of the photodiode portion is increased, the dark currents increase and they also vary to a large extent.
These dark currents are probably produced by an increase in crystal defects present in a depletion layer as a result of an increase in impurity concentration of the p-type layer in the junction between p- and n-type regions, and the junction between p- and n-type regions formed to suppress dark currents at a semiconductor/oxide layer interface, or are probably produced by a high electric field locally produced in the vicinity of the junction.
When the impurity concentration of the p-type region is decreased to suppress the dark currents, the contact resistance in the p-type region increases, and its variation also becomes large, resulting in larger characteristics variations of photoelectric conversion apparatuses.
That is, upon application of a general micropatterning technique to the prior art, problems of increases in dark current and variations of dark currents are posed.